1. Field of the Invention
This invention relates generally to a process for making acrylic coating resins and more specifically to superior solvents useful in the synthesis of high solids acrylic coating resins.
2. Description of the Prior Art
A large variety of acrylic coating compositions are known. Low solids coatings, i.e., those containing about 18 to 40 wt.% solids and the balance solvents, have heretofore been developed in which the resins themselves are characterized by high molecular weights, e.g., molecular weights in the range of 20,000 to 40,000. Such high solvent concentrations are required with these high molecular weight resins in order to supply flowability and other properties necessary for ease in applying a uniform coating. Due to strict air pollution regulations, pollution abatement of solvents is of paramount importance. To this end, the industry has expended much effort in an attempt to develop electrostatically sprayable coatings containing high solids contents, that is, coatings having a lower amount of solvents in order to satisfy pollution regulations. Attempts to achieve high solids coatings by merely using more of the conventional high molecular weight resins in the coatings have not been successful since the increased solids content using these resins results in an unacceptably high viscosity, and often the larger amounts of the resins cannot themselves be dissolved. Efforts at developing a "super solvent" for these conventional high molecular weight resins have also not proved to be successful. Therefore, coatings containing low molecular weight resins (e.g., for about 1,000 to 7,000 weight average molecular weight) would be desirable in high solids coatings in order to reduce the amount of solvents necessary in the blending for coating applications and, hence, the pollution difficulties associated with the solvents themselves. These high solids acrylic coatings are useful as exterior finish for automobiles, trucks, metal furniture, and as an appliance finish.
In U.S. Pat. No. 3,267,057, a polar solvent of high dielectric constant (greater than 8, preferably from 20-40) is employed with oil modified alkyd resins. However, adequate leveling was not achieved by these inventors in spraying of 67 wt.% solids coatings containing butyl acetate when sprayed by electrostatic atomization.
U.S. Pat. No. 3,348,965 discloses alkyd resin compositions which are capable of being sprayed electrostatically and incorporates polar organic solvents (butyl acetate is so classified) in its sprayable composition to increase conductivities.
U.S. Pat. No. 3,796,590 is directed to acrylic solution coating compositions which can be applied by electrostatic spraying employing solids contents of from 40 to 65 wt.% and organic solvents comprising aromatic hydrocarbon solvents alone or in combination with certain alcohols. Portions of butyl acetate are indicated as also being suitably present.
German Patent Publication No. 1,519,258 contains a disclosure (no examples) to the use of esters as polar solvents in electrostatic sprayable alkyd compositions, which was apparently added to the lacquer by the German patentees to decrease resistivities. The sprayable composition contained from about 45-55% by weight solids.
K. K. Mitra, "Electrostatic Application of Paint", Paint India, vol. 29, no. 9, pp. 52-56 (1979) indicates that while non-polar solvents (aliphatic and aromatic hydrocarbons chlorinated solvents, terpenes, etc.) can be employed in electrostatically sprayable paints to increase bulk and resistivity, polar solvents are used to control resistivity. The polar group is said to include ketones, alcohols, esters, ethers, ether alcohols, and nitro paraffins.
Also illustrative of prior art, high solids acrylic resins are those disclosed in U.S. Pat. No. 4,276,212 and in European Patent Applications Nos. 27,719; 29,339; 29,594 and 29,683.
Solvents which are indicated to be typical in these references (e.g., those mentioned in European Patent Application No. 29,594) are: Toluene, xylene, butyl acetate, acetone, methyl isobutyl ketone, methyl amyl ketone, methyl ethyl ketone, butyl alcohol and other aliphatic, cycloaliphatic and aromatic hydrocarbons, esters, ethers, ketones, and alcohols.
U.S. Pat. No. 4,151,152 discloses fast setting polyurethane varnishes which can be applied by airless or electrostatic spraying, and discloses esters as suitable high boiling varnish solvents. Included as suitable esters are hexyl acetate, iso-amyl acetate and cyclohexyl acetate. Highly concentrated solutions are prepared containing from 50 to 70 percent solids which must then be diluted prior to use.
Also relevant is a brochure entitled "Hexyl Acetate for the Coatings Industry" (Enjay Chemical Company), published prior to 1980, which discloses the use of hexyl acetate as coating solvent in certain specific low solids acrylic coating compositions; in urethane coatings; in nitrocellulose coatings; and in baking enamels.